1. Field of the Invention
The present invention relates to cylinder head cooling structure for a four-cycle air-cooled internal combustion engine for a vehicle such as a motorcycle. The cylinder head cooling structure includes an oil temperature control system for cooling oil that, if necessary, circulates to cool a high temperature portion in an internal combustion engine by circulating the oil to an oil cooler according to the temperature of the oil. The cooling structure further includes an arrangement for an oil temperature sensor that makes it possible to precisely detect a cooling system oil temperature as a representative value of the temperature of an air-cooled internal combustion engine.
2. Description of the Background Art
In general, in a four-cycle air-cooled internal combustion engine, high-temperature portions are cooled by the heat radiation operation of cooling fins formed on the surface thereof. Since, however, the periphery of spark plug mounting holes and the periphery of combustion chamber-side openings of intake ports and exhaust ports are within the internal combustion engine, sufficient cooling cannot be performed by the heat radiation operation of cooling fins on the outer peripheral portions of the internal combustion engine. For this reason, an engine has been developed in which oil jackets are provided around the spark plug mounting holes, and a portion of lubricating oil is circulated via an oil passage for cooling. Such an engine is disclosed, for example, in FIG. 1 of Japanese Utility Model Laid-Open No. S61(1986)-32512.
In this example, the oil passage has been formed between the intake port and the exhaust port in a direction orthogonal to the ports so as to intersect a vehicle advancing direction at right angles. Therefore, the periphery of the spark plug mounting holes is cooled by the oil, but the periphery of the intake port and the exhaust port, which have high heat loads, is not sufficiently cooled.
It is an object of the present invention to provide structure in which not only the periphery of the spark plug mounting holes, but also the peripheries of the intake port and the exhaust port is sufficiently cooled by oil.
Conventionally, a thermostat (temperature sensing valve assembly) is integrated into the housing of an oil filter, and the housing is attached to the front of an internal combustion engine. This configuration is disclosed, for example, in FIGS. 1 and 2 Japanese Patent Laid-Open No. 2000-34915.
In the conventional configuration, since the thermostat is integrated with the oil filter and is attached to the front of the engine, it is difficult to maintain the oil filter, and the external appearance of the engine is degraded. In addition, cooling system oil and lubricating system oil have been allowed to flow together to a thermostat. In terms of oil used for cooling an internal combustion engine, therefore, the oil temperature control for a portion intended to be cooled tends to slow.
In a water-cooled internal combustion engine, the cooling water temperature is detected and used as a representative temperature of the internal combustion engine. However, in an air-cooled internal combustion engine, the temperature of oil circulating in the internal combustion engine is detected and used as a representative temperature of the internal combustion engine. In the prior art, an engine is disclosed in which a temperature sensor is arranged with an oil returning path, the oil returning path formed in a cylinder block in order to return oil supplied to a cylinder head to an oil pan. An oil temperature detected by the temperature sensor is used as a representative value of the internal combustion engine temperature. This configuration is disclosed, for example, in FIGS. 2 and 4 of Japanese Patent Laid-Open No. 2000-213326.
In this example, because the oil returning path is provided rather leftward at a lower portion of the cylinder block, the temperature sensor is arranged on the left side of the lower portion of the cylinder block. As a result, a body portion of the temperature sensor projects outwardly from the cylinder block. Further, the position at which the temperature sensor is attached in the example described above is a position which becomes the front face side of the internal combustion engine depending upon the angle of attachment of the internal combustion engine to a vehicle.
When a temperature sensor is arranged on an internal combustion engine for a small size vehicle, if the temperature sensor is arranged on a side face of a cylinder, then the width of the internal combustion engine increases. On the other hand, if the temperature sensor is arranged on a front face of the cylinder, and since the detection value of the sensor is varied by an influence of being exposed to water or the like, a protective member is required, and then the cost increases. Further, since an engine control unit (ECU), which uses the detected internal combustion temperature to control fuel injection and/or ignition, is usually placed rearward of the internal combustion engine, if the temperature sensor is provided downwardly or forwardly of the cylinder, then the wires from the temperature sensor to the ECU become undesirably long.
The present invention contemplates provision of an oil temperature sensor arrangement structure for an internal combustion engine in which exposure of the sensor to water is avoided and in which long wiring lines are not required.